Three major glaciers in Alaska and Washington state have thinned and shrunk dramatically, clear signs of a warming climate, according to a study released Thursday by the U.S. Geological Survey.

The three glaciers -- Gulkana and Wolverine in Alaska and South Cascade in Washington -- are considered benchmarks for those in alpine and maritime climates because they closely parallel other glaciers in their regions. They have also been the subject of close scientific scrutiny since 1957.

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"These are the three glaciers in North America that have the longest record of mass change," said Shad O'Neel, a United States Geological Survey glaciologist in Anchorage who was one of the study authors.

"All three of them have a different climate from the other two, yet all three are showing a similar pattern of behavior, and that behavior is mass loss."

Scientists are keeping a close watch on melting glaciers, as a rise in sea-levels would threaten coastal and low-lying areas around the world.

The latest study compares records of snow and ice thickness and densities over the years, the factors used to calculate mass. The glaciers have lost mass as melting outpaced new snow and ice accumulation, and for all three, the losses were especially dramatic over the past 15 years, according to the USGS study.